Asia - Fitness level for hill tribe treks?

This is more a general impressions question, but I was wondering about those day-long hill tribe treks around Chiang Mai that are group tours. Usually they combine a bit of trekking with things like elephant rides, bamboo rafting, and such.

What I'm wondering if, how do you tell how 'fit' you have to be for the treks? I'm a very overweight female but I do work out pretty regularly despite that. I'll go on weekend 2-3 hour hike with no problem over rolling terrain, once or twice a month. Granted, I don't hike briskly, but I move along. There's a 'mini mountain' near me, Kennesaw Mountain, that's about 1000 feet rise over a mile's trail with some steep stretches, and I can climb that -- just a tad slower than others and with a few stops along the way after steep stretches.

How do I tell if a hill tribe trek is going to be too challenging for me, though?

In general, tourists on hill tribe treks are assumed to be completely out of shape, and the descriptions of the treks are usually adjusted to fit. For example, I went with a group of 4 on the "demanding" 2-day Aneks Trek (http://www.chiangdao.com/nest/2daytreks.htm) "for the fit and adventurous", tagged with the warning "this trek is definitely not suitable for all". My hiking nut friend complained that it was way too easy and she didn't even work up a sweat; a friend with a desk job and I both found it a little sweaty in parts, but much easier than we'd expected given then hype; and the least athletic member of our group wheezed and puffed a bit but didn't have major problems keeping up. Treks like this (http://www.chiangdao.com/nest/r21graftingtrek.htm), on the other hand, can probably be completed by anybody not on a respirator.

All that, though, a word of caution: Thailand is hot and muggy, even up north, and jungle trekking is very sweaty work if you're not acclimatized to it.

I took a trek in Laos and found it to be all of the difficulty advertised and THEN some; there was a 6 hr steep grade incline in the morning of the first day. I was alone with my guide and we took it slow because it was hard on both of us. I would do it again, though; a truly memorable experience of the highest caliber.